Currently you can't enter MX Records, CNAME, A Records or TXT records ahead of time before migrating a custom domain. This means there is a period where DNS is not accurate and could caused failed mail delivery or other services from not working. It also requires me to hang around at the computer and wait for the Heroic servers to detect the DNS propagation to open up the record-configuration process and then enter the records manually in more of a "time-crunch" situation.